The present invention relates to an encoder output error detecting device for monitoring the differences between first and second signals in the form of a set of sine and cosine wave signals produced, for instance, by a photoelectric encoder. More particularly, the invention relates to such a detecting device which detects the differences in DC component, amplitude and phase between the two signals and the difference are controlled so as to not exceed certain tolerances.
Almost all the general purpose encoders (incremental encoders) commercially available are relatively low in accuracy, generating pulses with a scale pitch divided by a factor of four to eight. In such an encoder, the angle measurement accuracy is determined by the encoder scale manufacturing accuracy and by the adjustments performed after assembly. The ordinary conventional encoder is not provided with any output error detecting device.
On the other hand, some high-accuracy encoders have output error monitoring devices, as disclosed, for instance, in Japanese Laid-Open Patent Application No. 169027/1983.
The conventional device employs a method in which, in order to detect an output abnormality of a first signal, the first signal is compared with a reference signal at the rise of the second signal, and output abnormalities of the second signal and similarly detected using the first signal. However, this method whereby in order to detect the output abnormality of the first signal, the data of the second signal is utilized, and vice versa, suffers from the drawback that abnormalities cannot be detected if the two signals are greatly shifted in phase from each other or one of the signals is too low in level to provide a rectangular wave signal.